Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine in part due to the engine being shut down during times when the engine operates inefficiently, or is not otherwise needed to propel the vehicle. During these conditions, the vehicle is transitioned from an engine mode to an electric mode where the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine is restarted. Vehicle propulsion is then transitioned from an electric mode to an engine mode.
One method of enabling a smooth engine restart in an HEV powertrain is disclosed by Tulpule et al. in US 20140088805. Therein, a disconnect clutch is disposed between an engine and a motor, which is operable to disconnect the engine from the motor. During an engine restart, the disconnect clutch is disengaged so that the engine can be fueled to obtain a speed that matches the motor speed. Then, when the engine speed matches the motor speed, the disconnect clutch is engaged to couple the engine and the motor to the drive shaft to meet the driver torque demand. In another example disclosed by Sah et al. in U.S. Pat. No. 8,628,451, engine speed and transmission input speed is synchronized when an oncoming clutch is activated and an outgoing clutch is deactivated. However the inventors have recognized potential issues with such an approach. As an example, if there is any speed difference between the engine and the impeller (or motor) speed, there may be substantial driveline disturbance and NVH caused when the disconnect clutch is closed. As such, there may be a difficulty in predicting the target speed at which the engine speed controller achieves synchronous speed for the disconnect clutch to close. This difficulty arises from the motor being used to propel the vehicle while the engine is being restarted, which results in the motor speed changing constantly. For example, while the engine is at 150-200 rpm, the motor speed may be as low as 600-700 rpm or as high as 2000 rpm. Predicting the target speed may become more difficult if a transmission shift is requested during the engine restart. For example, if a driver requests increased acceleration while the engine is being restarted, the transmission may command a downshift concurrent with the engine restart. If the engine speed control is targeted to the higher speed of the gear existing when the engine restart was initiated, a higher level of airflow and fuel may be commanded to accelerate the engine quickly to the higher speed. If the transmission changes to the lower gear of the transmission shift at an inopportune time, the engine speed may overshoot the motor speed in the reduced gear and lead to significant driveline disturbance. This can result in vehicle surge and NVH issues. In the same way, if a driver requests decreased acceleration while the engine is being restarted, the transmission may command an upshift concurrent with the engine restart. If the engine speed control is targeted to the lower speed of the gear existing when the engine restart was initiated, a lower level of airflow and fuel may be commanded to accelerate the engine quickly to the lower speed. If the transmission changes to the higher gear at an inopportune time, there is a high likelihood that the engine speed will undershoot the motor speed in the higher gear and lead to significant driveline disturbance. As such, this can result in vehicle stall and NVH issues.
The inventors have recognized these issues and developed a method for a hybrid vehicle with an improved engine restart method. In one example, a driveline method comprises: during engine starting of a moving vehicle, the engine starting during a transmission shift transition, adjusting an engine speed based on a future gear of the transmission shift; and closing a disconnect clutch before completion of the transmission shift. In this way, engine speed can be controlled to a synchronous speed based on the future gear, reducing driveline disturbances.
As an example, while a vehicle is propelled via motor torque from an electric motor, an engine restart request may be received. Accordingly, the engine may be cranked via the electric motor with a disconnect clutch coupled between the engine and motor at least partially open. Following cranking, engine fueling may be resumed and the engine speed may be controlled to a synchronous speed after which the disconnect clutch may be closed. If the engine is restarted during a transmission shift transition, the engine speed may be controlled to match a transmission input shaft speed based on the future gear of the transmission following the transmission shift. For example, if the engine is restarted during a transmission downshift from a first, higher gear to a second, lower gear, the downshift commanded due to the operator requesting acceleration during the engine restart, the vehicle controller may adjust engine parameters to control the engine speed profile towards the lower transmission input shaft speed expected in the second gear, rather than the higher transmission input shaft speed expected in the first gear. Then, when the engine speed matches the synchronous speed, the disconnect clutch may be closed, and thereafter the transmission shift may be completed (e.g., the second gear may be engaged). This allows the engine speed to not undershoot the required synchronous speed at the time of disconnect clutch closing, reducing NVH issues.
In this way, a quality of engine restarts in a hybrid electric vehicle, such as those performed concurrent to a transmission shift, may be improved. By controlling the engine speed during a run-up phase of an engine restart so as to better match a future gear of the transmission shift, rather than a current gear, NVH issues and driveline torque disturbances associated with speed underestimation or overestimation can be reduced. By predicting a synchronous speed expected at the time of disconnect clutch closing based on the nature of the transmission shift, vehicle stalls and bumps may be averted. Overall, a smoother engine restart with reduced NVH issues is enabled, improving operator drive experience.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.